This invention relates to lithographic printing plates and to a method for their production. More specifically, it relates to lithographic printing plates made from organic polymeric compositions either alone or in combination with a metal or a polymer base, More specifically still, it relates to the production of printing plates made from a metal and an organic polymeric composition by engraving the polymeric surface of the printing plate with a focused laser beam.
Lithography, in contrast to letter-press printing from raised surfaces bearing ink or gravure printing from depressions containing ink, is the process of printing from surfaces that are essentially flat. The contrast between the image and background is not dependent on a difference in height between the two portions of the plate, but rather on their differing ability to accept ink or water. Lithographic plate making, then, is the process of forming an imge or design on a plate which will take a greasy ink, and treating the other non-imaging areas of the plate so that they will take water and, when moist, will refuse ink.
Lithographic plates come in a variety of forms, from etched, metal plates to type embossed paper and plastic mimeograph sheets, and the processes for producing these plates are as varied as their form. Photolithographic plates, however, consisting of a metal base and a thin coating of a light sensitive organic polymerizable material, are closest in kind to the present invention. In photolithography, a metal plate, often an aluminum plate, which accepts water, is coated with a light sensitive, unpolymerized, organic composition, such as a diazo compound, an azido compound or a photopolymerizable polymer which will polymerize upon exposure to light, and which, in its hardened state, will accept ink and reject water. The surface of the plate is exposed to light through a negative or a mask and that portion of the surface exposed to the light hardens. The other unpolymerized portions can be washed off the surface of the plate by some suitable solvent. In use, the plate is usually first moistened with water, so that when the ink is applied to the plate, the ink will be rejected by the metal and accepted by the polymerized coating which is not readily wetted by water. The ink may then be transferred directly or by offset to the surface to be imaged or printed.
Such plates are durable enough to be useful in moderately long printing runs and are reasonably inexpensive. One of their failings, however, is the fact that the imaged portions of such plates usually account for only a small portion of their surface. The larger metal, non-imaged portions will rub on the inking and transfer rollers and cause rapid wear to these parts. To avoid this, lithographic plates are often made from materials such as copper, which have been found to accept ink, and coatings which will accept water instead of ink. The background region is then polymerized and the image area washed away. The ink is accepted by the metal base and rejected by the moist polymer. Once again printing is by direct or ofset transfer, but because the majority of the surface of the plate is plastic, not metal, less wear on the rollers is experienced. The plates themselves, however, are no more durable than those made from aluminum, and are more expensive because copper has been substituted for aluminum.
The system described above is quite versatile. By judicious selection of the materials used, both positive and negative plates can be produced, and photographic production of the plates is faster and less expensive than hand production. There is room for improvement, however. For one thing, the use of a metal base plate, especially one made from copper is expensive, so there is a need for plates made entirely from plastic, or ones made with inexpensive metals such a aluminum. In the second plate, photolithography requires plates which must be kept unexposed during storage and which, for use on a commercial scale, requires massive and expensive equipment for photoimaging and developing. There is need for a simpler system. Finally, in those instances where the information to be imaged on the plate is in the form of electronic impulses, the conversion of these impulses to photographic negatives prior to the formation of the printing plates is an expensive and time consuming step. There is need for a system to directly convert such electronic signals into lithographic printing plates.
It is an object of the present invention, therefore, to provide a process for producing lithographic printing plates by other than photopolymerization. It is a further object of the present invention to provide a process for the production of lithographic plates which is less expensive than that presently in use. It is a still further object of the present invention to provide a process for the formation of lithographic plates directly from electronic impulses.